Acid copper plating



1955 G. w. JERNSTEDT ETAL 2,700,019

ACID COPPER PLATING Filed July 5, 1951 Acid Copper Electrolyte Plus z-Thiohydontoin Compound us on additional Agent.

Fig.2.

SM m m Y I d. mm. e V. mw m, n A w .wM 1 mm V| M 8 mm B m i s 0. n .0..m m m C mm y m mm X m .t 9. .YO W. F E 0 L 28 3- Y B C A n o c e S O me .w m m m 9n m .m m e m5 0 m s s x m s e E A W W v O B 0 3.2.60 230 IUnited States Patent ACID COPPER PLATING George W. Jernstedt and MyronCeresa, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation,East Pittsburgh, Pa., a corporation of Pennsylvania Application July 5,1951, Serial No. 235,135

Claims. (Cl. 204-52) advantages are encountered in using them. Many ofthese addition agents must be employed in such minute amounts in theacid copper electrolyte thaLsatisfactOry determinations of the quantitypresent are almost impossible to make and it requires guesswork on thepart of the operator either to determine how much to add to begin withor to replenish the amount present to some desired optimum amount.Furthermore, the effect of most previously employed addition agents hasbeen relatively meager so that a high level of consistently good resultshas been difficult to obtain in acid copper plating.

One disadvantage in acid copper plating as carried out at the presenttime is the necessity for staying within a restricted range oftemperatures during plating within which the electrolytes must be keptto produce satisfactory plating. In commercial installations of acidcopper plating baths at the present time, temperatures of approximately100 F. are regarded as the maximum permissible using known additionagents in the baths. Artificial cooling of the bath is practiced to keepthe temperature below this value. and higher present day acid copperplating baths produce definitely inferior copper plate which isordinarily regarded as inacceptable.

Copper plated from acid copper plating electrolytes containing any ofthe number of addition agents now used in industry today tends to bequite brittle. This brittleness is undesirable in that it makes themachining or other processing of members plated with such copper quitedifiicult. Also striations or ribbing are commonly encountered whileplating from acid copper electrolytes containing these previously knownaddition agents.

The object of the present invention is to provide an acid copperelectrolyte containing thiohydantoin and certain substitutionderivatives of 2-thiohydantoin.

A further object of the invention is to provide-a process forelectroplating copper from an acid copper electrolyte containingthiohydantoin or certain substitution derivatives of 2-thiohydantoin.

Another object of the invention is to provide-an addition agent for acidcopper baths comprising essentially thiohydantoin or certainsubstitution derivatives of. 2- thiohydantoin, with or without otheradditives.

A still further object of the invention is to provide an acid copperelectroplating electrolyte with an addition agent comprising2-thiohydantoin and certain derivatives of 2-thiohydantoin incombination with either organic carboxylic acids or with dextrin orboth.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description anddrawing, in which:

Figure l is a view in cross section, and

Fig. 2 is a graph. We have discovered that the addition of either2-thiohydantoin or substitution derivatives of 2-thiohydantoin or two ormore of such compounds, to acid copper electrolytes will enableelectrodeposits with a highly refined grain size and having smooth,bright surfaces to be plated from the electrolytes so treated. Many ofthe dis- At temperatures of 120 F.

r6 ce advantages encountered with additions of previously known additionagents to the same electrolytes are eliminated by the addition of these2-thiohydantoin compounds.

More particularly, the present invention is based on the addition ofpredetermined amounts of at least one 2-thiohydantoin compound having atleast one organic snbstituent in any one of the 1, 3 and 5 positions.The base of these compounds is the following thiohydantoin structure:

H O (i-- 4 5 I 3 1 2 /N. C

Various organic radicals may be substituted at any one or more of the 1,3, or 5 positions, so long as the com- 1 pound is not rendered soinsoluble that it will not dissolve in the acid copper electrolyte in anamount sufficient to produce an appreciable result. Examples of suitablecompounds are:

2-thiohydantoin l-acetyl-Z-thiohydantoin5(2-hydroxylbenzal)-2-thiohydantoin S-furfural-Z-thiohydantoinl-benzoyl-2-thiohydantoin S-benzal-Z-thiohydantoin1-methyl-2-thiohydantoin 3-acetyl-2-thiohydantoin The best results havebeen secured with Z-thiohydantoin derivatives having a carbonyl group..C ll 0 attached at any one or all of the 1, 3 and 5 positions, andwith an organic radical being attached to the carbonyl group.

We have found as little as 0.0005 ounce per gallon of one or more ofthese Z-thiohydantoin derivatives will produce an improvement in theacid copper electrolyte. In some cases as much as 0.05 ounce per gallonof the compounds may be added. It will be appreciated that the relativesolubility of the compounds will determine the maximum amount that maybe applied. Extremely good results have been secured with1-acety1-2-thiohydantoin and specific reference will be made hereinafterto this compound, though, it will be understood that others may besubstituted in whole or in part therefor.

Acid copper electrolytes suitable for plating copper are well known.Ordinarily they comprise an aqueous solution having dissolved thereinfrom 20 to 33 ounces per gallon of copper sulfate and from 1.3 to 13ounces per gallon of sulfuric acid. In industry at the present time, theacid copper bath most widely used is one comprising a solution of 27.5ounces per gallon of copper sulfate crystals and 6 ounces per gallon'ofsulfuric acid (98% Into the acid copper electrolyte there may be addedl-acetyl- 2-thiohydantoin in an amount of from 0.0005 to 0.05 ounce pergallon. The optimum proportions appear to be from 0.005 to 0.01 ounceper gallon. It will be appreciated that as the electrolyte is used inplating the addition agent will require replenishing from time to time.When. treated with acetyl thiohydantoin, very satisfactory copperplating can be done with the bath at any temperature from roomtemperature up to 150 F. We have secured fine smooth copper depositsfrom bathsoperating at temperatures of R, which appears to be theoptimum temperature.

One or more of the 2-thiohydantoin compounds may be employed in the bathalone or in combination with other known addition agents. For example,We have added l-acetyl-Z-thiohydantoin to acid copper bathscontainingphenol sulphonic acid, thiourea, glue, metallic addition agents such ascadmium and the like. In every case, we have secured better copperelectrodepositsbyi the addition of the l-acetyl-Z-thiohydantoin to eachof these baths.

As a result of numerous tests run under comparable conditions, both inthe laboratories and in the shop, we have found thatl-acetyl-Z-thiohydantoin, for instance, enables the brightest copper tobe deposited that we have secured with any single known addition agent,and furtherrlnore the electrodeposited copper is relatively nonbritt e.

In order to enable the l-acetyl-2-thiohydantoin, for instance, tofunction at the maximum efficiency for long periods of time, it isdesirable to associate it with an organic carboxyl acid in amounts of upto about 3.00 ounces per gallon of the electrolyte or up to the limit ofsolubility for the less soluble carboxylic acids. The organic carboxylacid should be water soluble and not decompose in the acid electrolyte.Examples of suitable organic carboxyl acids are:

Adipic acid Phthalic acid Malic acid Linoleic acid Pimelic acid, andAconitic acid A number of these acids may be added if desired.Hereinafter specific reference will be made to citric acid, but it willbe understood that one or more of the other water soluble organic acidsmay be substituted in whole or in part therefor.

We have found further that the addition of dextrin to the acid copperelectrolytes containing 2-thiohydantoin and its 1, 3 and 5 derivatives,with or without a carboxylic acid, enables further benefits to beobtained. The dextrin prolongs the period of effectiveness of the 2-thiohydantoin additives in the electrolyte. The combination in an acidcopper electrolyte of a 2-thiohydautoin compound, a carboxylic acid suchas citric acid or aconitic acid, and dextrin has given the optimumquality in copper plating, and the most uniform plating for the longestperiods have been produced from this combination. The amount of dextrinfor best results is from 0.1 to 1.0 ounce per gallon of electrolyte,though as little as 0.01 ounce and as much as 5 ounces per gallon ofelectrolyte constitute an efiective amount.

Acid copper aqueous electroplating electrolytes withl-acetyl-Z-thiohydantoin added thereto, with or Without citric acid, anddextrin, or any other additive, may be employed for plating metal bypassing either continuous direct current or periodically reversedelectrical current or other suitable electrical current therethrough.Excellent results have been obtained with direct current plating fromsuch electrolytes. However, periodic reverse current has givenoutstanding electrodeposits characterized by an absence of nodules, andhaving smooth edges, smooth surfaces and refined grain not attainablewith direct current.

Referring to the drawing there is illustrated in Fig. 1 an apparatus forpracticing the present invention. This apparatus comprises a tank 12provided with a suitable liner 14 of rubber, glass or the like,resistant to the acid electrolyte, carrying an electrolyte 16 composedof an aqueous solution of copper sulfate, sulfuric acid and at least one2-thiohydantoin derivative as described herein. Disposed within theelectrolyte is an anode 18 that may be composed of copper or lead, orseparate anodes of both. If lead anodes are used, the copper must bereplenished by introducing copper sulfate into the electrolyte 16 fromtime to time. The anode 18 is suspended from a conductor bar 20. A base22 to be plated with copper is suspended by a support 24 from a secondconductor bar 26. The conductor bars 20 and 26 are provided withelectrical current from a suitable source 28 which may be a generator, arectifier, storage batteries or the like. Electrical current passingfrom the source 28 to the conductor bars 20 and 26 passes through theanode 18, electrolyte 16 and the base 22 to cause copper to be depositedfrom the electrolyte upon the base. The l.-acetyl-2-thiohydantoin willcause the copper to be deposited as a smooth bright layer substantiallyfree from brittleness and striation or ribbing. The copper depositedwill exhibit a highly refined grain and will be 'superior to copperdeposited from any known acid copper electrolyte bath.

Copper may be plated from the electrolyte by means of a reversedelectrical current composed of cycles, each of which passes electricalcurrent through the base for a period of time of from 0.01 second to 100seconds to plate copper on the base and then the direction of flow ofthe current is reversed to deplate a part of the previously deplatedcopper. The time and the magnitude of the deplating current is such thatit applies from 8% to of the coulombs applied during the previousplating period. Assuming efficiency during the deplating period, thismeans that from 8% to 90% of the copper deposited during the previousplating period in each cycle is deplated. The increment of copperremaining on the base after the cycle consists of smooth, sound copperupon which a second layer of copper is plated by the plating portion 'ofthe next cycle of period reversed current and then a portion of thissecond increment is deplated by passing of deplating current leaving asecond increment of still smoother copper than the first increment, andso on.

Referring to Fig. 2 of the drawing, there is illustrated in a graph theperiod reverse current as it is applied to the base. It is assumed thatthe base when first immersed in the electrolyte is at a zero potentialso that no current flows. When the first cycle of periodically reversedcurrent is applied, a cathodic or plating current of a density of thevalue A is applied and metal is plated for a period of time X to a pointB, then the direction of flow of the current is reversed so that thecurrent density in the member drops from the value B to zero and thenbecomes anodic and will deplate copper, reaching a deplating currentdensity of C. Metal is deplated for a period of time Y, which is atleast 6 of the length of period X, at the current density of C to Duntil sufficient coulombs of deplating current have been applied toequal from 8% to 90% of the coulombs applied during the plating periodX. The cycle A-B-C-D deposits an increment of sound, smooth copper onthe base. The direction of current flow is again reversed from D throughzero and then plating current of a density value of F is applied tobegin another cycle which will plate a second increment of copper. Itwill be understood that the showing in Fig. 2 is merely schematic andthat the current is not necessarily uniform from A to B or C to D, asshown, but will vary and be relatively non-uniform. Also in reversingfrom B to C and from D to F the time required is finite and these lineswill not be vertical, as shown, but will take an appreciable period oftime, depending upon the various factors involved in the platinginstallation. The deplating or anodic current density C-D may be equalto the plating current density A-B, or exceed it or may be as low as 35%of the plating current density. Reference should be had to Patents2,451,- 341 and 2,470,775 for additional information as to periodicreverse current cycles.

The following examples are illustrative of the practice of theinvention:

Example I An aqueous electroplating electrolyte of the followingcomposition was prepared:

Ounces per gallon Copper sulfate (crystals) [CuSO4.5H2O] 27.5

Sulfuric acid (98%) 6 1-acetyl-2-thiohydantoin l 0.007

This bath was operated at various temperatures from 60 F. to 150? F.with excellent results. Both temperatures of from F. to F. appeared togive optimum plating results.

Copper was plated from the bath of this Example I us ing direct currentat current densities of from 50 to 100 amperes ,per square foot. In eachcase the copper had a highly refined grain and was quite bright.

Copper was plated from the bath of Example I using a periodic reversecurrent having the following cycles:

Plating time: Deplating time, seconds (a) 2 seconds /3 (b) 5 seconds (c)10 seconds 2 (d) '15 seconds 3 The current density during each portionof the cycles (1:) and (b) was 50 amperes per square foot and 60 amperesper square foot for cycles (0) and (d). The periodic reverse currentcycles in each case produced excellent smooth deposits of copper betterthan anything securedunder the same conditions using many other additionagents previously known in the art.

In another test 3-acetyl-2-thiohydantoin was used instead ofl-acetyI-Z-thiohydantoin in this Example I. The plating solutionproduced copper deposits fully equivalent to those described in ExampleI.

Example II To the bath of Example I there was added 0.25 ounce pergallon of citric acid. A base immersed in the bath was plated byapplying continuous direct current thereto. The deposited copper wasbright "and showed a fine grain structure without any ribbing orstriations. The citric acid enabled good plating to be obtained forlonger periods than possible with the bath of Example I without citricacid. In another test a periodic reverse current comprising a cycle of20 seconds plating and seconds deplating applied at a current density of75 amperes per square foot during both portions of the cycle produced onmembers excellent bright copper deposits free from any surface defects.

Example 111 An electrolyte having the following composition wasprepared:

Copper was plated from this electrolyte with both direct current and aperiodic reverse current having cycles with 15 seconds plating periodand 3 seconds deplating perlod. The deposits of copper were brighterthan from the electrolyte without the dextrin.

To six separate portions of the electrolyte of this Example III, therewas added 0.25 ounce per gallon of citric acid, malic acid, maleic acid,linoleic acid, oxalic acid and aconitic acid, respectively. Theseportions were used in plating over a period of many days. Throughoutthis period the plated copper was of excellent color and characterizedby a smooth, extremely fine grain structure.

We have found it to be desirable to prepare a composition by combiningthe Z-thiohydantoin compounds as disclosed herein with either a watersoluble organic car boxylic acid or dextrin, or both, and copper sulfatemay be included, which composition may be added to water and sulfuricacid to prepare the bath originally and to replenish the bath asrequired from time to time. Sultable compositions of this type compriseessentially at least 0.1% by weight of at least one Z-thiohydantoincompound or derivatives thereof having at least 1 organic substituent atthe l, 3 and the 5 positions, not exceeding 80% by weight of at least 1water soluble organic carboxylic acid or dextrin or both, and thebalance being copper sulfate crystals, or other additive such asthiourea or metal salts. An example of such composition to be added tothe electrolyte is the following:

Example IV The following in powdered form were admixed:

Parts by weight l-acetyl-Z-thiohydantoin l1 Citric acid 1475 Coppersulfate crystals 13,170

This composition was added in the amount of 0.25 ounce per gallon ofacid copper electrolyte and would provide therein 0.002 ounce per gallonof l-acetyl-2-thiohydantoin.

Since certain changes may be made in the above invention and differentembodiments of the invention may be made Without departing from thescope hereof, it is intended that all matter contained in the disclosureshall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. An aqueous electroplating electrolyte comprising essentially coppersulfate, sulfuric acid, from 0.0005 to 0.05 ounce per gallon of at leastone 2-thiohydantoin compound and from efiective amounts up to 3.0 ouncesper gallon of an organic carboxylic acid soluble in water, the organiccarboxylic acid being selected from the group consisting of citric acid,malic acid, maleic acid, linoleic acid, oxalic acid, adipic acid, andaconitic acid.

2. An aqueous electroplating electrolyte comprising es-' sentiallycopper sulfate, sulfuric acid, from 0.0005 to 0.05 ounce per gallon ofat least one 2-thiohydantoin compound from effective amounts, up to 3ounces per gallon of an organic carboxylic acid soluble in water, theorganic carboxylic acid being selected from the group consisting ofcitric acid, malic acid, maleic acid, linoleic acid, oxalic acid, adipicacid, and aconitic acid, and from 0.1 to 5 ounces per gallon of dextrin.

In the process of plating on a base copper from an aqueous electrolyte,the electrolyte comprising as its essential ingredients copper sulfateand sulfuric acid, the steps comprising adding from 0.0005 to 0.05 ounceper gallon of at least one Z-thiohydantoin compound and dextrin in anamount of from 0.1 to 5 ounces per gallon and then passing a platingelectrical current from an anode through the electrolyte and to the baseto deposit copper on the base.

4. The process of claim 3 wherein up to 3 ounces per gallon of at leastone water soluble organic carboxylic acid is added to the electrolyte,the organic carboxylic acid being selected from the group consisting ofcitric acid, malic acid, maleic acid, linoleic acid, oxalic acid, adipicacid, and aconitic acid.

5. In the process of plating on a base copper from an aqueouselectrolyte, the electrolyte comprising as its essential ingredientscopper sulfate and sulfuric acid, the steps comprising adding 0.0005 to0.05 ounce per gallon of at least one 2-thiohydantoin compound and up to3.0 ounces per gallon of at least one water soluble organic carboxylicacid, the organic carboxylic acid being selected from the groupconsisting of citric acid, malic acid, maleic acid, linoleic acid,oxalic acid, adipic acid, and aconitic acid, and then passing anelectrical current from an anode, through the electrolyte and to thebase to deposit copper on the base.

6. In the process of plating on a base copper from an aqueouselectrolyte, the electrolyte comprising as its essential ingredientscopper sulfate and sulfuric acid, the steps comprising adding from0.0005 to 0.05 ounce per gallon of at least one 2-thiohydantoin compoundselected from the group consisting of 2-thiohydantoin and itsderivatives having at least one organic radical substituted at the l, 3and 5 positions, and from effective amounts up to 3.0 ounces per gallonof at least one water soluble organic carboxylic acid, the organiccarboxylic acid being selected from the group consisting of citric acid,malic acid, maleic acid, linoleic acid, oxalic acid, adipic acid, andaconitic acid, and then passing cycles of periodically reversedelectrical current through the base, the aqueous electrolyte and ananode, each cycle of current first flowing in one direction toplate'copper on the base for a period of time of from 0.01 second to 100seconds, then the direction of current flow reversing for a period oftime to deplate a portion of the previously plated copper, the coulombsapplied during the deplating period equal to from 8% to 90% of thecoulombs applied during the plating period, the plurality of cycles ofperiodically reversed current electrodepositing smooth sound copper onthe base.

7. An addition agent composition to be added to acid copper electrolytescomprising essentially a mixture of at least 0.1% by weight of at leastone 2-thiohydantoin compound and a substantial amount but not exceeding80% by weight of at least one Water soluble organic carboxylic acid, theorganic carboxylic acid being selected from the group consisting ofcitric acid, malic acid, maleic acig, linoleic acid, oxalic acid, adipicacid, and aconitic ac1 8. An addition agent composition to be added toacid copper electrolytes comprising essentially a mixture of at least0.1% by weight of acetyl 2-thiohydantoin compound, and a substantialamount but not exceeding 80% by weight of citric acid.

9. An addition agent composition to be added to acid copper electrolytescomprising essentially about 118 parts by weight of at least oneZ-thiohydantoin compound having at least one organic substituent at thel, 3 and 5 positions, about 1475 parts by weight of citric acid andvabout 13,170 parts of powdered copper sulfate crystals.

10. An addition agent composition to be added to acid copper platingelectrolytes comprising essentially a mixdantoin compound, and asubstantial amount but not exceeding 80% by weight of a mixture ofdextrin and at 7. least one water soluble organic carbo'xylic acid, theorganic carboxylic acid being selected from the group consisting ofcitric acid, malic acid, maleic acid, linoleic 'acid, oxalic acid,adipic acid, 'and acon'itic acid.

References Cited in the file of this patent UNITED STATES PATENTS2,391,289 Beaver Dec. 18, 1945 8 Wilsfin Nov. 26, 1 946 Jem'st'edt Oct.12, 1948 Phillips et 31. n. Aug. 7, 1 951 FOREIGN PATENTS Canada Nov.22, 1949

2. AN AQUEOUS ELECTROPLATING ELECTROLYTE COMPRISING ESSENTIALLY COPPERSULFATE, SULFURIC ACID, FROM 0.0005 TO 0.05 OUNCE PER GALLON OF AT LEASTONE 2-THIOHYDANTOIN COMPOUND FROM EFFECTIVE AMOUNTS, UP TO 3 OUNCES PERGALLON OF AN ORGANIC CARBOZYLIC ACID SOLUBLE IN WATER, THE ORGANICCARBOXYLIC ACID BEING SELECTED FROM THE GROUP CONSISTING OF CITRIC ACID,MALIC ACID, MALEIC ACID, LINOLEIC ACID, OXALIC ACID, ADIPIC ACID, ANDACONITIC ACID, AND FROM 0.1 TO 5 OUNCES PER GALLON OF DEXTRIN.